Scanning near-field optical microscopy is used to measure the mode profile and evanescent field of a Ge-doped D-shaped optical fiber. The structure of the fiber is determined by differential etching followed by an investigation of the resultant topography with an atomic force microscope. This information is then used to theoretically model the expected behavior of the fiber and it is shown that the theoretical results are in excellent agreement with the experimentally observed field. ©1997 American Institute of Physics.